Plastic containers that can reliably contain carbonated beverages generating internal pressures as high as 100 psi or more, and that can be inexpensively manufactured in attractive shapes, pose technical problems that have received substantial attention.
The spherical shape, which has the greatest ratio of volume to surface area, provides an optimum uniform distribution of wall stresses generated by internal pressures. Thus, the spherical shape achieves the maximum reliable and effective strength for a given wall material thickness. Indeed, internal pressures within non-spherically-shaped containers tend to urge the non-spherically-shaped containers toward a spherical shape. A spherical shape is, however, unacceptable as a commercial beverage container because, among other obvious reasons, a sphere has no stable base, is difficult to handle, and cannot effectively use shelf and storage space of retail and wholesale purveyors and manufacturers.
An extensive variety of cylindrical plastic beverage containers have been designed that can reliably and attractively contain carbonated beverage products. Generally, the commercial containers can be easily handled, can be inexpensively manufactured, and have stability when filled and unfilled. Early designs for cylindrical containers employed the advantages of the spherical shape by employing a hemispherical bottom to which a separate base cup was added to provide stability. Cost considerations have largely replaced such designs with one-piece cylindrical containers having one of two general designs.
One design for commercial containers includes a "champagne" type base including concave, or "domed" evasion-resisting central bottom portions merging with the cylindrical container sidewalls at an annular ring which forms a stable base for the container. Unfortunately, champagne bases require a greater wall thickness in the base portion to resist the distending and everting forces of the internal pressure, particularly during hot summer months. Additionally, stress concentrations at the annular base-forming transition between the concave central bottom portion and cylindrical sidewall make the base portion prone to stress cracking and rupture when the container is dropped. One container using this general champagne base design is disclosed in U.S. Pat. No. 4,249,666.
Another design for commercial containers employs a plurality of feet protruding downward from a generally convex web structure joining the sidewall of the container to a central bottom portion. Many container designs providing footed bottles are in commercial usage. Examples of containers using this design are disclosed in U.S. Pat. Nos. 4,865,206 and 5,353,954. Such containers have most frequently been manufactured from plastic materials such as polyethylene terephthalate (PET) by blow molding a preformed parison into a mold formed in the shape of the container. The biaxial expansion of PET by blow molding imparts strength to the formed PET material. Blow molded PET can provide economically acceptable containers with minimal wall thicknesses. Such containers typically exhibit sufficient strength to contain pressures up to 100 psi and more, and resistance to gas permeation that can deplete the carbonation from the contained beverages. An important performance criterion for footed bottles is the maintenance of the lowest point on the axis of the container above the supporting surface. This is achieved by ensuring that the lowest point on the feet of the container remains below the lowest point on the axis over all pressures that the container is likely to face. However, some containers of the prior art do not satisfy this performance criterion at the pressures commonly developed within filled containers stored at ambient temperatures on hot summer days.
One factor that is frequently over looked in container designs is the propensity of PET to succumb to the deleterious effects of stress cracking and crazing. Stress cracking and crazing is manifest as almost imperceptible streaks in the plastic but ultimately can become complete cracks due to stress and environmental factors. Harmful environmental factors include the exposure to stress cracking agents such as caustics, water, oils and generally any plastic solvent or softening agent. Relatively unstretched portions of a plastic container, such as the central bottom portion, that have low degrees of crystallinity due to the lack of biaxial expansion are particularly susceptible to crazing and stress cracking. The relatively unstretched central portion of the container bottom is generally integrally joined to a plurality of depending feet that are formed with distention-resistant but stress concentrating areas. The composite effect on such areas of stress and strain due to the internal pressure of the container and external environmental factors can lead to crazing, stress cracking and container bottom failure. Efforts to improve the design of such footed containers have frequently led to bottom portions including small radii of curvature, discontinuities, and abrupt transitions between adjoining surfaces that provide additional stress concentration, crazing and stress cracking sites. Additionally, such footed containers frequently exhibit creases and folds in the bottom of the feet detracting from the appearance of the container and possibly even contributing to increasing instability or failure of the container. While many of the known designs are in wide commercial use, none of these container designs is entirely satisfactory in view of cost, manufacturability and reliability.
The desired plastic container for carbonated beverages would exhibit low cost and weight, and would be manufacturable from plastic material by blow molding with minimal plastic material. The desired container would also exhibit a maximal volume with minimal total height in an easily handled diameter. The desired container would also exhibit maximal sidewall height to provide large surface area for product labeling. The desired container would also exhibit excellent stability in both filled and unfilled conditions over a wide range of temperatures and pressures. The desired container would also exhibit a freedom from high stress concentrations, crazing and stress cracking.